Laser beam welding method

ABSTRACT

The invention relates to a method of producing a welding seam with a single laser pulse, the laser pulse and the work piece being moved relative to each other at a high velocity. This results in the length of the welding seam formed being primarily the product of pulse duration of the laser pulse and the relative velocity of work piece and laser pulse. The invention also relates to a method in which the work piece moved at high velocity relative to the laser beam is subjected along the welding seam to be formed to repeated applications of a single laser pulse. In that case the length of the welding seam is defined principally of the product of pulse duration of the laser pulse and the relative velocity of the work piece and laser pulse divided by the number of applications.

TECHNICAL FIELD

[0001] The invention relates to a method of welding of work pieces bypulsed laser radiation, in particular of components of precision andmicro system technology, in accordance with the preamble of claim 1. Thejoining of components in the field of horology and, in this context, thewelding of clock or watch components such as pinions, crowns and axlesof mechanical clocks or watches is a preferred field of application.

[0002] A further field of application is the electric industry, forinstance in the area of sensors equipped with diaphragms for measuringgas pressures or temperatures. Such sensors require especially thindiaphragms reliably welded to the remaining components.

[0003] Furthermore, the invention may be applied in the area ofautomotive industry, for instance for joining rotationally symmetriccomponents of a diameter less than 5 mm, for instance components of fuelfeed devices for combustion engines or for connecting hubs to shafts.Plug connections may be produced as well by the welding method inaccordance with the invention, particularly such plugs which havehitherto typically been soldered with a lead-containing solder. Plugs ofcopper and brass may be produced particularly well. In addition, theinvention is particularly well suited for welding foil capacitors inview of the fact that low energy application is required for theprotection of the dielectric disposed between the foils which has notheretofore been achievable by conventional resistance gap welding.

[0004] The method according to the invention is furthermore suitable forwelding together components of different materials such as, inparticular, for welding together steel and brass.

THE STATE OF THE PRIOR ART

[0005] In the field of macro technology, laser beam welding is generallyknown, and this contexts it constitutes a major field of application oflasers. Thus, DE 3,820,848 A1 discloses a welding method with pulsedlaser radiation the intensity of which is controlled for forming at theconnection site a laser-induced plasma as a function of one or moreconnection site parameters.

[0006] Furthermore, EP 0,440,002 B1 describes a device for spot weldingby which body components of an automotive vehicle are welded by means ofa multiple-joint robot arm. In this arrangement, the parts to be weldedmove by the welding robot arm at low velocity and are welded thereby bea plurality of spot welds.

[0007] EP ,0923,424 B1 teaches the use of a movably mounted beamdeflection unit with a stationary laser welding head, which by aplurality of spot welds yields a corresponding number of line weldswhich are spatially separated from each other.

[0008] Aside from the selection of pulsed laser radiation for thewelding of macro components as in the previously mentioned publications,welding by continuous laser radiation is also known. In general, thisresults in a more uniform application of energy into the components tobe welded and, therefore, to lower distortion. A further advantageresides in the reduced soiling of the components by liquified orvaporized material.

[0009] Furthermore, it is generally known that processing andfabrication methods in the field of macro technology cannot usually betransferred to the field of micro systems technology. Based upon thespecific conditions of the micro systems technology, new or speciallyadapted processes are often required. Because of their small dimensions,components of mechanical watches are typically joined by adhesive orpress joining methods, for instance.

[0010] The adhesive method requires pre and post processing steps,however, and the components have to b cleaned in a time-consumingmanner. Moreover, a further processing steps is necessary, i.e. theapplication of the adhesive. These requirements significantly prolongthe fabrication process. In addition, in respect of the intendedadhesive connect ion, the time between application of the adhesive andconnection of the parts is critical.

[0011] In the case of pressed connections, narrow mechanical tolerancesmust be maintained which limit the application of press joiningtechnology. The press joining technology requires the application ofmechanical force. However, this force subjects the components todeformations and distortions. Thus, it is often difficult to adhere tothe required low tolerances.

PRESENTATION OF THE INVENTION

[0012] The invention is based upon the technical problem of providing analternative connection process which results in particularly lowdistortions of the connected components.

[0013] The technical problem is solved by the characteristics of theindependent claim. Advantageous improvements are defined by thedependent claims.

[0014] The invention led to the realization that the technical problemmentioned above can be solved by a welding method by which the weldingseam is formed by a single laser pulse having a pulse duration T_(H),and in which the length l of the welding seam, relative to the directionof welding movement, is primarily determined by the product of the feedvelocity v and pulse duration T_(H).

[0015] The invention is based upon the realization that at a relativevelocity between the laser beam and the work piece the resultant weldingspot on the work piece is longer than it would be without the relativemovement.

[0016] Without such high relative velocity between the work piece andthe laser beam, the length l of the welding seam, relative to thedirection of forward movement of the weld, such as, for instance, theforward movement of the laser beam, is typically larger by a factor of1.5 to 2 than the extent of the width b_(o) of the welding seam relativeto the direction. By way of approximation, it can be said that for thecase of low relative velocity the two values are equal, i.e. I≈b_(o).

[0017] Moreover, where the relative velocity between work piece andlaser beam is zero, using a rotationally symmetric beam, the length ofthe welding seam I equals the width of the welding seam b_(o), i.e.I=b_(o) if v=0. Since in most cases operations are carried out withrotationally symmetric beams, in the context of the present inventionand in the absence of relative velocity between work piece and laserbeam, the parameter welding seam width b_(o) in general also defines thelength of the welding seam in the forward welding direction, regardlessof the laser beam being actually rotationally symmetric. Of course, themethod in accordance with the invention may be practiced regardless ofthe shape of the laser beam.

[0018] In conventional spot welding and in connection with work piecesof steel, given a beam diameter of the laser radiation on the work piecerelative to, or measured with respect of the laser beam advancemovement, of 200 μm, the length of the welding seam in this directionalso measures about 200 μm. Hence, for a desired welding seam length inthe millimeter or centimeter range, a sequence of overlapping weldingspots is required.

[0019] Where the relative velocity v between work piece and laser beamis greater than zero, the beam diameter on the work piece is extended bythe distance v x t, t being the time during which the work piece issubjected to radiation. Where one laser pulse is applied, the time tequals the pulse duration T_(H). Thus, the entire length of the weldingseam is I=b_(o)+v×T_(H).

[0020] Placing a single welding spot by the conventional spot weldingmethod, the length l of the welding seam of this single welding spot isdominated by the diameter of the beam or, in other words, the termv×T_(H) is small relative to the width of the welding seam b_(o)dominated by the diameter of the welding seam. The opposite is true inthe method in accordance with the invention, where at welding seamlength l the term v×T_(H) dominates or where the length of the weldingseam I is primarily determined by the advance of the beam diameter onthe work piece.

[0021] The result is that in the advance direction of the laser beam thewelding seam being formed is longer than it would be in the absence of acorresponding large relative velocity. Preferably, the product ofadvance velocity v and pulse duration T_(H) is greater than the extentof the width b_(o) of the welding seam on the work piece relative to thedirection of welding advance, by a factor of at least η_(s)=3, and, moreparticularly, by factor η_(s)=10. Obviously, η_(s) is the quotient fromthe product of the advance velocity and the pulse length divided by thewidth b_(o) of the welding seam, i.e. η₂=(v*T_(H))/b_(o).

[0022] The relative velocity v between laser beam and work piece to beselected in the an actual application is thus determined by the quotientof the desired length l of the welding seam and the pulse durationT_(H). Therefore, at a desired welding seam length l of 1 mm and at apulse duration of 20 ms the relative velocity to be selected would be 3m/min.

[0023] In the case of multiple application of the welding seam, therequired relative velocity must be multiplied by the number ofapplications.

[0024] The relative velocity between laser beam and work piece may beselected to be as high as 50 m/min. This ensures particularly longwelding seams in view of the fact that the length of the welding seamobtained is the result of the product of relative velocity v and pulseduration T_(H). Where, for instance, the pulse duration T_(H)=20 ms, thewidth of the welding seam b_(o)=0.4 mm and the relative velocity v=50m/min, it is possible, with the method according to the invention, toattain a welding seam length l=17.1 mm with a single laser pulse.Furthermore, as a result of the high relative velocity the energy of thepulse may be applied over a particularly long distance and therefore ata particularly low distance energy yielding components of even lesserdistortion. Such high relative velocity results in a factor of η_(s)>40.It was thus possible to achieve satisfactory results in welding copperand brass.

[0025] If a relative velocity in excess of 50 m/min is selected theoccurrence of such physical effects as humping may be expected.Therefore, depending upon the given material, the maximum possiblevelocity in connection with the method in accordance with the inventionshould be limited to 50 to 60 m/min. Where these effects are lesspronounced, higher relative velocities may be selected.

[0026] The application of continuous wave radiation or pulsed laserradiation as a sequence of pulses are avoided by the welding method inaccordance with the invention. As a consequence, the total energyapplied to a component is particularly low resulting in acorrespondingly low distortion of the component. Relative toconventional pulsed laser radiation of macro components, the applicationof energy is lower by a factor of about 10.

[0027] A further advantage of the welding method in accordance with theinvention is that when welding alloys, the welding seam is less subjectto fissures and displays fewer pores. It is assumed that the improvedquality of the welding seem is the result of the fact that in the methodin accordance with the invention the material is melted only once,whereas in spot welding with overlapping welding spots the material ismelted repeatedly. However, during multiple melting more time isavailable to the material during which its alloy components can separateor during which one or more of the components of the alloy may vaporize.In the single melting of the method in accordance with the invention,the operation is too quick to result in the formation of a significantseparation.

[0028] Moreover, the process provides for particularly clean welding,i.e. no contaminations occur as a result of particles from the weldingarea. A problem of such contaminations occurs, for instance, whenwelding work pieces of brass, i.e. an alloy CuZn37. The melting point ofthis alloy is 920° C., and the vaporizing temperature of zinc is about908° C. The application of high energy in conventional pulsed weldingmethods results in zinc vaporizing from the alloy. When welding with themethod in accordance with the invention, experiments displayed no zincevaporation, and the welded work pieces of this alloy remain clean.Probably, ass set forth in the preceding paragraph, the reason for thisresides in the single melting allowing no substantial proportion of thezinc to separate and to be present in elementary state.

[0029] It has generally been found that the method in accordance withthe invention is particularly advantageous in connection with metallicmaterials. The reason for this resides in the generally high heatconductivity of metallic materials. The high heat conductivity requiresoperating at high power levels so that as high a proportion as possibleof the energy applied serves to create a locally limited melt bathwithout being distributed in the entire work piece. In addition, theabsorption of copper, for instance, of the applied radiation is verylow. For that reason, some of the metallic materials, for instance brassand copper, can be treated with continuous wave lasers on a limitedscale only, for the power generated by the latter is in generalsignificantly lower than the power of the pulses of pulsed lasers.

[0030] Since the medium output power of pulsed laser radiation is low,the laser radiation, compared to a CW laser of the same power, is of ahigher radiation quality, and it is possible, therefore, to focus to asmaller diameter at the same operating distance. In this manner it ispossible to form welding seams of a width <100 μm, which are barelydiscernible to the naked eye. This is important for connections inproducts, such as watches and clocks, which require a high degree ofvisual appeal, or where a flawless appearance is important to induce acustomer to purchase a product.

[0031] Compared to conventional press-joining, gluing and crimpingmethods, laser welding methods offer the further advantage of aparticularly short processing time, in view of the fact that suchprocess steps as cleaning of the components after joining can beavoided. This makes the processing of the components more costefficient. Moreover, compared to known laser welding methods, practicingthe method in accordance with the invention results in a processing timelower by a factor of 10. Furthermore, the known laser welding methodsinject so much energy into the components leading to heating smallercomponents such that they would be unacceptably distorted. Not until themethod of the present invention has it been possible to weld of smallcomponents with a laser at advantages over known joining techniques.

[0032] The laser pulse may be provided by a conventional pulsed laser ofa low median laser power sufficient for the method in accordance withthe invention. Compared to continuous wave laser beam sources, theadvantage of the pulsed laser beam source is that its investment andmaintenance costs as well as corresponding costs for the requiredcooling aggregates are especially low because of its low average outputpower. Alternatively, a lamp-pumped of diode-pumped CW-laser may bechosen provided laser pulses can be set at a pulse duration <100 ms.

[0033] Processing with particularly low distortions of components may beachieved by selecting the relative velocity between work piece and laserradiation sufficiently high so that for producing a single welding seamthe components are sequentially subjected several times to a singlelaser pulse along the welding seam to be formed. In this manner, theentire energy is substantially simultaneously injected into the entirewelding seam to be formed. The resultant practically simultaneousheating of the components along the welding seam leads to a lowerdistortion than does the chronologically sequential heating of thecomponents along the welding seam. In this alternative embodiment of theinvention the length of the welding seam is primarily determined by theproduct of the advancement velocity v and the length T_(H) of the pulsedivided by the number of applications.

[0034] This alternative embodiment of the invention is especially usefulin connection with small rotationally symmetric components which may berotated at high speeds. Moreover, in rotationally symmetric componentsthe almost simultaneous heating results in a particular reduction ofdistortions.

[0035] In a further embodiment of the invention the power of the laserpulse may be altered over time. Such a chronological change in theoutput power of the pulse or chronological change of the pulse formationmakes it possible to take into consideration the change in opticalproperties of the material as may occur during processing. Thus, inlaser welding the degree of coupling is strongly dependant upon theabsorption of the material. Also, the oxidation of the surface and theroughness of the surface affect the welding operation. The absorption isalso subject to the temperature of the material and in general increaseswith rising temperatures. Changing conditions along the length of thewelding seam may thus be compensated by changes in the formation of thepulse. The penetration depth of the weld and the width of the seam aswell as the quality of the surface of the welding seam may also beinfluenced by the pulse formation.

[0036] For further improving the quality of the welding seam, a detectormay detect characteristic radiation from the welding site, such asreflected radiation or heat radiation, and the power of the laser may beadjusted in accordance with the signals received by the detector duringprocessing. Since the signals received by the detector are primarilydependent upon temperature, the laser power may be adjusted as afunction of with temperature. As referred to in the preceding paragraph,this adjustment of the laser power results in an optimized energyinjection into the work piece and accordingly to an improved result ofthe weld.

[0037] It is, of course, possible to provide several welding seams, suchas a “basting seam”, in one component by the welding method inaccordance with the invention.

[0038] The relative velocity between the laser beam and the componentsto be welded may take place either by moving the processing optics, bymoving the components, or by moving both the processing optics and thecomponents. The choice is dependent upon the geometry of the componentsand the required joints. The set relative velocity depends upon the typeof movement, such as linear or rotational, as is limited by suchphysical effects as humping.

[0039] A further improvement of the welding seams may be obtained byrelative movement between work piece and laser beam such that the laserbeam is aimed as precisely as possible at the desired welding seam, i.e.the seam is being traced. This may be accomplished by an evaluationunit, e.g. a camera or a light section device which evaluates the siteat which the laser beam is to impinge upon the work piece. The relativemovement between work piece and laser beam may be controlled as afunction of this evaluation. It is of particular advantage during thewelding operation to detect the position on the work impinged by thelaser beam. By comparing the actual impinged position against thedesired position impinged by the laser beam, the relative movementbetween work piece and laser beam may be appropriately adjusted so thatthe laser beam impinges as precisely as possible on the work piece alongthe desired welding seam. In this manner, positioning errors of thecomponents to be welded may be adjusted and higher tolerances inpositioning the components to be welded may be accepted. Without thementioned measures the permissible tolerances are less than 10% of thewidth b_(o) of the welding seam.

[0040] With stationary components, the laser beam may be moved. This isoften advantageous in view of the fact that the laser beam may usuallybe moved with greater precision than the components. Moreover, movementof the components may result in an undesirable shifting of thecomponents to be welded relative to each other.

[0041] In accordance with FIG. 1a, processing of the components may takeplace by directing the beam of a laser 1 against the component 6 by wayof a beam spreader 2 a and 2 b, a deflector 4 and the processing optics5. The component 6 is positioned on a moveable positioning system 7. Thepositioning system may be a rotational plate or a linearly moveabletable. Combinations of rotational plate and linearly moveable table maybe provided as well. Accordingly, linear movements, rotary movements aswell as combinations of rotational and linear movements may be achieved.By these movements it is possible to provide linear welding seams,rotationally symmetric welding seams and welding seams of any desiredshape.

[0042] Another possibility of providing relative movement between a workpiece and a laser beam is to move the laser beam. For this purpose,special optics such as a scanner 8 may be used as shown in FIG. 1b.

[0043] Instead of the rigid beam guidance of 1 a, a flexible light guide3 as shown in FIG. 1c, may be used. From it, the light enters theprocessing optics 5 whence it is directed against the component 6positioned on a motion imparting system 7.

[0044] A further possibility is to direct the laser pulse to thecomponent 6 supported by a system 7 moveable about four axes as shown inFIG. 2a. In this case, the component 6 may be rotated about an axisposition vertically positioned in the plane of the drawing as well asabout an axis positioned horizontally in the plane of the drawing, asindicated in FIG. 2a by bent arrows. In addition, the component may bemoved in a plane positioned vertically of the plane of the drawing asindicated in FIG. 2a by intersecting arrows.

[0045]FIG. 2b depicts the possibility of directing laser light through aflexible light guide 3 to the processing optics 5 and thence to thecomponent 6 moved by a four-axes positioning system 7. The choice of afour-axes positioning system makes it possible by a single laser pulseto form rotationally symmetric welding seams.

[0046] With light guidance identical to that of FIG. 1a, FIG. 3a depictsthe possibility of directing the laser light to a component 6 moved by arobot 9.

[0047]FIG. 3b depicts a variant of an industrial robot 9 which moves alight guide 3 for directing the laser light to predetermined locationsof the component 6 which are appropriately positioned by the movementsystem 7.

[0048] The invention will hereafter be described in greater detail withreference to embodiments.

[0049] In accordance with a first embodiment according to FIG. 4, watchcomponents are being welded. The shafts and crowns of mechanical watchesare shown in FIG. 4 and both of them consist of stainless steel. Thediameter of the shaft measures 0.9 mm, and the crown has a diameter of 5mm. During production, shaft and crown are made separately, and theshaft is then inserted into the cylindrical recess in the center of thecrown. In this instance, welding serves to provide a secure connectionbetween the shaft and the crown. Welding was carried out by a singlelaser pulse of 20 ms duration, a pulse power of 125 W at a pulse energyof 2.5 J. At a welding velocity of 8.5 m/min the energy of the distancewas 0.88 J/mm with the length of the welding seam being π×0.9 mm=2.8 mm.The structure of the watch components welded by the method in accordancewith the invention was of significantly higher precision and stabilityby comparison with adhesively produced watch components. Pre-treatmentsare no longer necessary, and usually the watch components may be usedwithout cleaning, particularly if welding takes place with a protectivegas for preventing soiling and oxidation. Furthermore, the forcerequired for pulling the components apart was increased by a factor of10 relative to adhesively connected components.

[0050] In a second embodiment, as shown in FIG. 5 a shaft of 0.3 mmdiameter was welded to a pinion of a mechanical watch. The shaftconsists of stainless steel, and the pinion consist of brass, e.i. aCuZn37 alloy. At a relative velocity of 2.83 m/min, welding was carriedout with a pulse energy of 1.3 J for the entire welding seam and a pulseduration of 20 ms. The distance energy was 1.38 J/mm.

[0051] In a third embodiment the welding of the second embodiment wascarried out at a relative velocity of 15 m/min. The length l of thewelding seam in this instance was equal to the circumference π×0.3mm=0.95 mm. Welding was performed with a single pulse of 4 ms durationat a pulse energy of 150 W×4 ms=0.6 J and a distance energy of 0.63J/mm.

[0052] In a fourth embodiment according to FIG. 6 a copper sheet of 0.2mm thickness was welded. A single welding seam has a seam width ofb_(o)=0.7 mm and a seam length l of 3.55 mm. It was formed by a singlepulse of a focal point diameter of 0.25 mm and a pulse duration of 20ms. The pulse energy was 25.8 J at a distance energy of 7.4 J/mm.

[0053] By subjecting the copper sheet to the same radiation source butconventional processing with 11 individual pulses of a pulse energy of21.6 J each, it was possible to obtain the same welding result asregards stability at the same welding seam length. The energy applied tothe work piece thus amounted to 237.6 J at a distance energy of 67.9J/mm. The pulse power of the used rectangular pulses was P_(H)=2,700 W,the pulse duration T_(H)=8 ms and the pulse repetition frequencyf_(p)=2.5 Hz. The feed velocity was 1.5 mm/sec; the required processingtime was 4.4 s. With the method in accordance with the invention theprocessing time is equal to the required pulse duration T_(H)=20 ms.Hence, the processing time of conventional welding is greater by afactor of 220. Furthermore, with conventional welding 9.2 times theamount of energy had to be applied to the work piece in order to yieldthe same stability at a welding seam of the same length. In addition,the seam requires a width of 0.75 mm. One reason for the slightlygreater seam width at the conventionally pulsed welding technique is theapplication of more energy and the resultant higher heating of the workpiece. This leads to a slightly larger area of the work piece beingmelted and to a larger welding seam.

[0054] List of Reference Characters

[0055]1 Source of laser beam

[0056]2 Beam spreader

[0057]3 Light guide

[0058]4 Beam guide

[0059]5 Processing optics

[0060]6 Component

[0061]7 Positioning or movement system

[0062]8 Scanner

[0063]9 Robot

1. Method of laser beam welding of a work piece, in particular forproducing components of precision and micro system technology, in whichthe laser beam and the work piece are moved relative to each other at avelocity v, characterized by the fact that a welding seam is produced bya single laser pulse of pulse duration T_(H), and that the length l ofthe welding seam is primarily defined by the product of the advancevelocity v and the pulse duration T_(H).
 2. Method of claim 1,characterized by the fact that the product of advance velocity V andpulse duration T_(H) is greater by at least factor 3, preferably factor10, than the extent of the width of the welding seam b_(o) on the workpiece.
 3. Method of laser welding of a work piece, particularly forproducing components for precision and micro system technology, in whichthe laser beam and the work piece are moved relative to each other byvelocity v, characterized by the fact that a welding seam is formed by asingle laser pulse of pulse duration T_(H) with the sequentiallyrepeated laser pulse being applied to the welding seam and the length lof the welding seam relative to the welding feed direction isprincipally defined by the product of feed velocity v and pulse durationT_(H) divided by the number of applications.
 4. Method of claim 3,characterized by the fact that the product of feed velocity v and pulseduration T_(H) is greater by at least by factor 3 divided by the numberof applications, preferably by factor 10 divided by the number ofapplications, than the extent of the width b_(o) of the welding seam onthe work piece relative to the welding feed direction.
 5. Method of oneor more of the preceding claims, characterized by the fact that thepower of the laser pulse is changed over time.
 6. Method of one or moreof the preceding claims, characterized by the fact that a detectordetects characteristic radiation from the welding site and that duringprocessing the power of the laser is adjusted as a function of theradiation detected by the detector.
 7. Method of one or more of thepreceding claims, characterized by the fact that several welding seamsare provided on a work piece.
 8. Method of one or more of the precedingclaims, characterized by the fact that the work piece and the laser beamexecute a linear movement and/or a rotational movement relative to eachother.
 9. Method of one or more of the preceding claims, characterizedby the fact that the relative movement between the work piece and thelaser beam is controlled as a function of a desired position of thelaser beam impinging on the work piece detected by an evaluation unit.10. Method of claim 9, characterized by the fact that the relativemovement between work piece and laser beam is controlled as a functionof an instant impingement position of the laser beam on the work piecedetected by an evaluation unit.
 11. Method of one or more of thepreceding claims, characterized by the fact that the relative movementbetween work piece and laser radiation is achieved by movement of thelaser radiation.
 12. Method of one of more of the preceding claims,characterized by the fact the welding seam is formed in an atmosphere ofprotective gas.
 13. Method of one or more of the preceding claims,characterized by the fact that watch components are being welded. 14.Method of one or more of the preceding claims, characterized by the factthat components of fuel feed devices for combustion engines or plugconnections are being welded.
 15. Method of one or more of the precedingclaims, characterized by the fact that foil capacitors are being welded.16. Components characterized by the fact that they are produced by amethod of one of claims 1 to 15.